Usagi and Mamoru share a very adorable relationship with each other, one that feels like new love... exciting and passionate and above all else, young. Haruka and Michiru's relationship strikes me as being a far more mature one, where they're so comfortable with each other that they're beyond jealousy and public displays of affection. They seem... well, cool. There's a certain air of mystery and maturity between them that makes them stand out and they're perfect complements to each other. Perhaps the grim nature of their initial mission matured their relationship, taking solace in each other in the face of the task ahead.
Haruka, as Michiru once explains, possesses the strengths of both genders. Her masculinity and brusque attitude is beautifully complemented by Michiru's gentle, feminine air. Michiru often keeps Haruka's cheek in check, scolding her when she acts conceited and rude... but also acts as Haruka's strength when she begins to doubt their purpose. It would seem that Michiru loves the soft side of Haruka; the side that wants to protect her loved ones at any cost. Similarly, it would seem that Haruka loves Michiru's gentle and focused nature.
Although Takeuchi Naoko, the creator of Pretty Guardian Sailor Moon, has confirmed that Haruka and Michiru are indeed in a relationship together, there are still people who deny it for a variety of reasons. Some are opposed to homosexuality, some believe that a lack of any physical intimacy or direct confession means there is no relationship at all, and some believe they are simply close friends. Of course, the negative attitude towards homosexuality has led the couple to be heavily censored in many countries. Some translations state that they are cousins and some state that they are simply friends. The attitude towards homosexuality is changing across the world, however, so it'll be interesting to see if Haruka and Michiru's relationship continue to be censored in new translations/localizations in the upcoming years.
Throughout the manga, we do see Haruka and Michiru embracing and even pulling away from a kiss at one point. The 90's anime has a considerable amount of evidence for their relationship; they enroll in a love contest and are winning until they decide to drop out, we see them holding hands in passionate moments, and we see them making suggestive comments toward each other that imply a sexual relationship between them. Those examples will be detailed in other sections of this fan site so please check them out if you're interested. ♥